1. Field of the Invention
The present invention relates to a heat fixing apparatus including a heating unit for heating a recording medium having a substrate, a fixing layer and a surface layer so as to sublimate sublimating ink applied in advance to the surface layer for transferring the sublimated ink to the fixing layer.
2. Description of the Related Art
An exemplary conventional technique relating to the above field of art is disclosed in Japanese patent application “Kokai” No: Hei. 10-297197. According to this, a metal substrate includes a coloring ground layer (corresponding to a “fixing layer” in the present invention) acting also as a rust-preventive layer, a transparent resin layer as an optical transparent resin layer formed over the coloring ground layer, the resin layer being made of acrylic resin, polyester resin, urethane resin etc., and an inkjet receiving layer (corresponding to a “surface layer” in the present invention) formed over the resin layer and made of e.g. porous alumina to form an ornamental metal body. After application of a sublimating ink or pigment on the inkjet receiving layer by an inkjet printing to form a colored pattern, the sublimating pigment is heated in a heating furnace or by a hot press, whereby the sublimating pigment in the inkjet receiving layer is sublimed into the transparent resin layer. As a result, the colored pattern formed in the ink receiving layer is formed in the transparent layer. Then, the inkjet receiving layer is removed to obtain an ornamental metal body having a colored pattern fixedly formed within the transparent resin layer.
According to an art presently under development, a recoding layer comprises a flexible resin sheet as the substrate, a fixing layer formed on the surface of this substrate and a surface layer (providing a similar function to the inkjet receiving layer described above) formed on the surface of the fixing layer, so that image information printed on the surface layer is subsequently heated to be transferred to and fixed in the fixing layer. Further, no apparatus has yet been developed which is capable of carrying out a continuous process for printing record information of the surface layer of such recording medium with sublimating ink and then heating the medium to sublimate the sublimating ink for transferring it onto the fixing layer.
Still further, Japanese patent application “Kokai” No: Hei. 10-16188 discloses an image forming apparatus. According to this, first, a primary image is formed on a thermal transfer sheet by e.g. an inkjet printer. Then, this thermal transfer sheet having the image formed thereon is laid over a recording sheet and these sheets are pressed and heated together, whereby the image (ink) formed on the thermal transfer sheet will be sublimed by the heat and transferred onto an ink fixing layer of the recording sheet, thus forming a secondary image thereon.
According to further art disclosed by Japanese patent application “Kokai” No: 2001-105638, sublimating ink is transferred from an ink ribbon onto a surface of a recording sheet. In order to heat and fix the ink on the sheet, the sheet is charged into a heater box, in which the sheet is advanced and heated between a press roll and a heat roll opposed to each other with a small gap therebetween or between a heat roll and a conveyer belt disposed along a portion of the peripheral face of the heat roll.
Further, in the field of textile printing, according to an exemplary technique disclosed by Japanese patent application “Kokai” No: Hei. 08-311782, dye is applied to a textile by the inkjet printing method. Then, in order to reinforce the fixing of the dye and also to improve its color development, the textile is charged into a heater device to be heated therein. Then, the textile is discharged from the device immediately to be cooled at the normal temperature.
With such apparatuses for effecting transfer in the recording medium by means of heating, the following inconveniences are expected to occur. For instance, in the case of using a flexible resin sheet as the substrate of the recording medium, when the heating takes place with irregularity in the apparatus with uneven force application, this will result in deformation in the substrate, so that the substrate will loose its flatness. Especially, when heating is effected in an irregular manner, the ink transfer will develop at an area having a high temperature whereas the ink transfer will be retarded or restricted in an area having a low temperature. This will lead to density and/or color irregularity of the transferred record information.
More particularly, a recording medium using fluororesin as the fixing layer is suitable for producing a product having superior weather resistance to products having image formed (printed) with silver-salt print paper or with an inkjet printer. Therefore, such product may be used as a billboard to be installed outdoors or a sticker or seal or the like for outdoor use. In using this type of recording medium outdoors, it will be effective to employ, as the substrate thereof, a PET (polyethylene terephthalate) resin sheet which is relatively inexpensive and has good water resistance as well as good flatness. This PET resin is thermoplastic. Hence, during heating of the recording medium, if the medium is heated while being pinched between and transported by heating press roller pair, unevenness if any in the pressing force from the press rollers or irregularity in the temperature distribution in the surfaces of the rollers will impair the flatness of the substrate, when in turn leads to density or color irregularity in the transferred image or the like. For this reason, there is a need for art allowing uniform heating.